1// Copyright 2014 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#include "components/rappor/rappor_metric.h"
6
7#include "base/logging.h"
8
9namespace rappor {
10
11RapporMetric::RapporMetric(const std::string& metric_name,
12                           const RapporParameters& parameters,
13                           int32_t cohort_seed)
14    : metric_name_(metric_name),
15      parameters_(parameters),
16      bloom_filter_(parameters.bloom_filter_size_bytes,
17                    parameters.bloom_filter_hash_function_count,
18                    (cohort_seed % parameters.num_cohorts) *
19                        parameters.bloom_filter_hash_function_count) {
20  DCHECK_GE(cohort_seed, 0);
21  DCHECK_LT(cohort_seed, RapporParameters::kMaxCohorts);
22}
23
24RapporMetric::~RapporMetric() {}
25
26void RapporMetric::AddSample(const std::string& str) {
27  bloom_filter_.AddString(str);
28}
29
30ByteVector RapporMetric::GetReport(const std::string& secret) const {
31  // Generate a deterministically random mask of fake data using the
32  // client's secret key + real data as a seed.  The inclusion of the secret
33  // in the seed avoids correlations between real and fake data.
34  // The seed isn't a human-readable string.
35  const std::string personalization_string = metric_name_ +
36      std::string(bytes().begin(), bytes().end());
37  HmacByteVectorGenerator hmac_generator(bytes().size(), secret,
38                                         personalization_string);
39  const ByteVector fake_mask =
40      hmac_generator.GetWeightedRandomByteVector(parameters().fake_prob);
41  ByteVector fake_bits =
42      hmac_generator.GetWeightedRandomByteVector(parameters().fake_one_prob);
43
44  // Redact most of the real data by replacing it with the fake data, hiding
45  // and limiting the amount of information an individual client reports on.
46  const ByteVector* fake_and_redacted_bits =
47      ByteVectorMerge(fake_mask, bytes(), &fake_bits);
48
49  // Generate biased coin flips for each bit.
50  ByteVectorGenerator coin_generator(bytes().size());
51  const ByteVector zero_coins =
52      coin_generator.GetWeightedRandomByteVector(parameters().zero_coin_prob);
53  ByteVector one_coins =
54      coin_generator.GetWeightedRandomByteVector(parameters().one_coin_prob);
55
56  // Create a randomized response report on the fake and redacted data, sending
57  // the outcome of flipping a zero coin for the zero bits in that data, and of
58  // flipping a one coin for the one bits in that data, as the final report.
59  return *ByteVectorMerge(*fake_and_redacted_bits, zero_coins, &one_coins);
60}
61
62}  // namespace rappor
63